Method for compensating overload trip curve for CT error

Abstract

A digital thermal model for compensating for error in a current transformer used in a solid-state overload relay. The thermal model implements a difference equation that determines using a low-pass filter two parameters corresponding to calibration points along an overload trip curve. The trip curve is adjusted at an ultimate trip current (one calibration point) independently of a trip time at a locked rotor current (another calibration point) of a motor protected by the overload relay. The ultimate trip current and trip time can be adjusted based on a motor full load current set by a user. Large CT error will cause the thermal model to adjust the trip time at the locked rotor current, increasing the range of acceptable CT error, allowing the overload relay to have a wider adjustment range.

What is claimed is: 
     
       1. A method of compensating an overload trip curve for error produced by a current transformer, comprising:
 compensating, using a controller, for the error at an ultimate trip current by adjusting the overload trip curve about a current axis corresponding to the ultimate trip current, the ultimate trip current defining a trip threshold that if exceeded causes an overload relay to trip responsive to an overload condition; 
 compensating, using the controller, for the error in a trip time at a locked rotor current of a motor connected to the overload relay by adjusting the overload trip curve along a time axis at the locked rotor current, the locked rotor current corresponding to a current for overcoming a locked rotor condition of the motor to cause the motor to begin to rotate; and 
 storing in a memory a representation of at least a portion of the compensated overload trip curve at the ultimate trip current and the trip time at the locked rotor current, 
 wherein the compensating for the error in the trip time includes: 
 the current transformer measuring an input current corresponding to the locked rotor current; 
 calculating a filter update period parameter, tau, as a function of the measured input current, the gain compensation parameter, and the trip time at the locked rotor current; and 
 updating a thermal model at the filter update period determined by tau. 
 
     
     
       2. The method of  claim 1 , wherein the compensating for the error at the ultimate trip current is carried out according to a function that includes a motor full load current (MFLC) corresponding to a maximum current drawn by the motor when delivering its full rated output power. 
     
     
       3. The method of  claim 2 , wherein the MFLC is adjustable by a user. 
     
     
       4. The method of  claim 1 , further comprising:
 causing the overload relay to trip responsive to a measured current exceeding the overload trip curve at the ultimate trip current; and 
 causing the overload relay to trip responsive to the measured current at the locked rotor current exceeding the overload trip curve for the trip time. 
 
     
     
       5. The method of  claim 1 , wherein the compensating for the error at the ultimate trip current is carried out independently of the compensating for the error in the trip time. 
     
     
       6. The method of  claim 1 , further comprising powering the controller by a power supply that also powers the overload relay, wherein the overload relay is a self-powered, solid-state overload relay. 
     
     
       7. The method of  claim 1 , wherein the compensated overload trip curve includes an asymptote that approaches the ultimate trip current at the current axis and intersects the locked rotor current at a compensated trip time along the time axis. 
     
     
       8. The method of  claim 1 , wherein the compensating for the error at the ultimate trip current includes:
 the current transformer measuring an input current corresponding to the ultimate trip current; 
 adjusting the input current by a fixed gain parameter to produce a gain compensation parameter; and 
 adjusting the gain compensation parameter based on a motor full load current (MFLC) corresponding to a maximum current drawn by the motor when delivering its full rated output power to produce a compensated trip threshold. 
 
     
     
       9. The method of  claim 8 , wherein the adjusting includes multiplying the input current by the fixed gain parameter. 
     
     
       10. The method of  claim 1 , further comprising:
 comparing an output of the thermal model with the compensated trip threshold; and 
 responsive to the output of the thermal model exceeding the compensated trip threshold, causing the overload relay to trip. 
 
     
     
       11. The method of  claim 1 , further comprising adjusting tau based on a motor full load current (MFLC) corresponding to a maximum current drawn by the motor when delivering its full rated output power. 
     
     
       12. A computer program product, comprising one or more non-transitory tangible media having a computer readable program logic embodied therein, the computer readable program logic configured to be executed to implement a method for compensating an overload trip curve for error produced by a current transformer, the method comprising:
 compensating, using a controller, for the error at an ultimate trip current by adjusting the overload trip curve about a current axis corresponding to the ultimate trip current, the ultimate trip current defining a trip threshold that if exceeded causes an overload relay to trip responsive to an overload condition; 
 compensating, using the controller, for the error in a trip time at a locked rotor current of a motor connected to the overload relay by adjusting the overload trip curve along a time axis at the locked rotor current, the locked rotor current corresponding to a current for overcoming a locked rotor condition of the motor to cause the motor to begin to rotate; and 
 storing in a memory a representation of at least a portion of the compensated overload trip curve at the ultimate trip current and the trip time at the locked rotor current, 
 wherein the compensating for the error in the trip time includes: 
 the current transformer measuring an input current corresponding to the locked rotor current; 
 calculating a filter update period parameter, tau, as a function of the measured input current, the gain compensation parameter, and the trip time at the locked rotor current; and 
 updating a thermal model at the filter update period determined by tau. 
 
     
     
       13. The computer program product of  claim 12 , wherein the compensating for the error at the ultimate trip current is carried out according to a function that includes a motor full load current (MFLC) corresponding to a maximum current drawn by the motor when delivering its full rated output power. 
     
     
       14. The computer program product of  claim 13 , the method further comprising:
 causing the overload relay to trip responsive to a measured current exceeding the overload trip curve at the ultimate trip current; and 
 causing the overload relay to trip responsive to the measured current at the locked rotor current exceeding the overload trip curve for the trip time. 
 
     
     
       15. The computer program product of  claim 13 , wherein the compensating for the error at the ultimate trip current is carried out independently of the compensating for the error in the trip time. 
     
     
       16. The computer program product of  claim 13 , wherein the compensated overload trip curve includes an asymptote that approaches the ultimate trip current at the current axis and intersects the locked rotor current at a compensated trip time along the time axis. 
     
     
       17. The computer program product of  claim 12 , wherein the compensating for the error at the ultimate trip current includes:
 the current transformer measuring an input current corresponding to the ultimate trip current; 
 adjusting the input current by a fixed gain parameter to produce a gain compensation parameter; and 
 adjusting the gain compensation parameter based on a motor full load current (MFLC) corresponding to a maximum current drawn by the motor when delivering its full rated output power to produce a compensated trip threshold. 
 
     
     
       18. The computer program product of  claim 12 , further comprising:
 comparing an output of the thermal model with the compensated trip threshold; and 
 responsive to the output of the thermal model exceeding the compensated trip threshold, causing the overload relay to trip.